Environmental concerns and regulations have spurred the need to reduce the amount of hazardous hydrocarbon vapors emitted by vehicles and have led to the development of new technologies for manufacturing plastic fuel tanks. A fuel tank constructed from a single layer of a plastic, such as high density polyethylene, has been found to have an unacceptably high permeation rate of fuel vapor therethrough. Therefore, current plastic fuel tanks are formed with multiple layers one of which is a vapor barrier layer and these tanks have reduced the amount of hydrocarbons released into the atmosphere by as much as 60 times over single layer untreated plastic fuel tanks.
A typical multilayer plastic fuel tank construction comprises an outer layer of high density polyethylene, an inner layer of high density polyethylene, and a vapor barrier layer disposed between them. The vapor barrier layer is typically a polymer such as ethylene vinyl alcohol which requires an adhesive layer adjacent both the outer and inner layers to join the high density polyethylene with the ethylene vinyl alcohol. A multilayer plastic fuel tank is more difficult to manufacture than a single layer plastic fuel tank. In mass production, to insure the quality of multilayer fuel tanks and the vapor barrier, it is important to determine the thickness of the individual layers of the multilayer structure and in particular the adhesive and barrier layers both from one tank to the next and at various locations within an individual tank. The most critical and difficult areas to measure tend to be at the corners, edges and other areas of the tank where the contour of the multilayer wall changes rapidly.
Previously, testing and sampling of manufactured plastic fuel tanks required cutting a cross section from various portions of the fuel tanks and preparing each cross section to be visually inspected under a microscope to measure the thickness of the various layers. This is undesirable because of the time required to prepare and inspect the sample cross sections of the fuel tank and also because of the destruction of the fuel tank as well as the scrap and waste created when conducting such a test.
U.S. Pat. No. 5,608,165 discloses a non-destructive method for determining the thickness of a barrier layer in a multilayer plastic fuel tank which utilizes an interference technique with a varying frequency and relatively low frequency and continuous ultrasonic signal to determine a frequency which produces an attenuated signal which can thereby be used to determine the thickness of only the vapor barrier.